Advances in technology and new demands on the existing Global Positioning System (GPS) led to an effort to modernize the GPS system. In 2000, U.S. Congress authorized the effort, referred to as GPS III. One of the key changes was the addition of two new L1 codes, taking the total signal codes transmitted by each GPS satellite from 3 to 5 signal codes per channel.
One of the key parameters of GPS navigation is ranging information. This information is obtained from the pseudo-random codes transmitted from the GPS satellites. A key characteristic of the pseudo-random codes is that they are orthogonal to each other, to the same codes from a different space vehicle, and to different time shifts of themselves. The GPS receiver independently generates local replicas of the pseudo-random codes in parallel as it receives the pseudo-random codes from a transmitting GPS satellite. The GPS receiver compares the generated codes with the received codes. When the time offsets of the corresponding code streams are aligned, correlation of the GPS signal with other GPS signals from other GPS satellites may be determined. Based on the correlated GPS signals, a GPS receiver may triangulate its position on the surface of the earth.
At present, each GPS satellite transmits a signal made up of three signal codes, these three signal codes are linearly combined and interplexed. To interplex three code signals, two of the code signals are added and placed on one phase of a signal, with the third binary signal and an additional throw-away signal being combined and placed on the other phase. The throw-away signal added to the solitary signal is completely defined by the other three pre-existing signals and contains no message information. Its primary purpose is to shift the symbol points of the signal linear constellation along a vertical axis so the symbols points are relocated onto an equi-amplitude circle. The amount each symbol point is shifted along a vertical axis is the same.
For a signal made up of three GPS signal codes, interplex is a relatively efficient approach, and provides for good correlatable Power Percentage, with the actual percentage of the radio frequency (RF) signal correlatable dependent on the code power ratios of the signal.
With the introduction of GPSIII and the additional two signal GPS codes, however, the current system of combining GPS codes can not be utilized, as the current method does not support the combining of the two extra GPS codes.